1. Field of the Invention
The present invention relates to a power supply circuit of switching regulator type and, more particularly, to an improvement thereof which may be used as a power source for supplying power to various electric appliances, and also may be used as a charger for charging a battery, e.g., nickel-cadmium battery.
2. Description of the Prior Art
The power supply circuit of switching regulator type is disclosed in the applicant's prior U.S. Pat. No. 4,488,210, and one example is shown in FIG. 1.
According to the prior art, the power supply circuit generally includes input rectifying portion 1, blocking oscillator portion 2, converter transformer 3, error detecting portion 4, control circuit portion 5, and output rectifying portion 6. When the power switch SW is turned on, a starting current Is flows from input rectifying portion 1 to the base of switching transistor TR4, thereby activating the blocking oscillator 2. When the operation of the power supply circuit becomes stable after its start, control circuit portion 5 controls the timing when to turn-off switching transistor TR4 in accordance with the signal obtained from error detecting portion 4. The operation under the stable condition is further described below.
Under the stable condition and when switching transistor TR4 turns on, in a manner which will be described later, a current Ii (FIG. 4, waveform B) flows through transistor TR4 and input winding N1. Thus the voltage across resistor R11 gradually becomes great in the negative direction in accordance with the time. At this time, capacitor C5 is already charged in the polarity shown in FIG. 1 in the period before switching transistor TR4 turns on, i.e., during the OFF-period of transistor TR4, by a current, Ir flowing from one end of winding N3. Thus, the voltage level V.sub.M at junction M, which is connected to the emitter of transistor TR2, is always negative (FIG. 4, waveform I), and is equal to the sum of voltage across resistor R11 and voltage across capacitor C5 added to the voltage level at line L0. Thus, during the ON-period of transistor TR4, the voltage level at junction M decreases, i.e., becomes great in the negative direction.
A junction N between resistors R7 and R8 connected in series between the collector of transistor TR1 and line L0, is connected to the base of transistor TR2. The voltage level at junction N is negative and is equal to the voltage V.sub.N (FIG. 4, waveform I) which is obtained by rectifying the voltage between taps i and k of winding N3 using diode D6 and capacitor C3 and comparing the rectified DC voltage across lines L0 and L1 with the voltage determined by zener diode ZD5 at transistor TR1. Therefore, when the voltage V.sub.M at junction M becomes lower than the voltage V.sub.N at junction N, control transistor TR2 turns on and, in turn, transistor TR3 turns on. Thus, a reverse bias current IB2 (FIG. 4 waveform E) flows through capacitor C5, resistor R11, emitter-base of switching transistor TR4, emitter-collector of control transistor TR3, resistor R10 and capacitor C5, thereby turning switching transistor TR4 off. Thereafter, transistor TR4 is maintained off by the reverse bias voltage obtained between taps i and j of winding N3.
Then, switching transistor TR4 is turned on in the following manner.
During the OFF-period of transistor TR4, a resonance operation takes place by the inductance and distributed capacitance of input winding N1. When the resonant current flows in the direction indicated by Ii, a positive feedback current If (FIG. 4, waveform E) is generated from winding N3 flowing from tap j, thereby turning switching transistor TR4 on. Transistor TR4 is maintained on until it is turned off in the above described manner.
By the on and off operations of switching transistor TR4, output winding N2 of converter transformer 3 generates a voltage having a rectangular wave (FIG. 4, waveform H). The voltage obtained during the OFF-period of transistor TR4 is rectified by diode D9 and capacitor C9, thereby producing a output DC voltage from output rectifying portion 6. Also, a voltage, which is proportional to the output DC voltage, is produced between lines L0 and L1. When this voltage varies due to the condition change of the connected load in the output rectifying portion 6 or the input voltage from the source, the voltage at junction N, i.e., the base voltage of transistor TR2, varies, thereby changing the time when to turn switching transistor TR4 off. Accordingly, the voltage across the load is maintained constant. For example, when the output DC voltage should increase, the voltage across lines L0 and L1 increases in a similar manner, thereby increasing the voltage level at junction N from a negative voltage to a voltage at line L0. Thus, the time when switching transistor TR4 turns off is made faster, making the ON-period shorter. Thus, decreasing the output DC voltage to the required level.
The prior art power supply circuit of FIG. 1 as described above has the following problems.
As shown in FIG. 2, the characteristics of load current to output DC voltage according to the prior art power supply circuit is such that the constant voltage control may be carried out when the load current is below a predetermined level, but the range for effecting the constant voltage control will vary relatively to the change of the input voltage from the source, as indicated by dotted lines. This is understood from the following description. In FIG. 2, a point A represents the maximum power producing point, at which the collector current of switching transistor TR4 becomes maximum value Icp (FIG. 4, waveform D). Since the maximum value Icp is determined by .beta. times the positive feedback current If of transistor TR4, i.e., base current IB1 (FIG. 4, waveform E) flowing during the ON-period and also the current IB1 is proportional to the input voltage applied across input winding N1, the range for effecting the constant voltage control will vary.
If the constant voltage control range changes with respect to the change of the input voltage in a manner described above, it is necessary to provide a switching transistor TR4 which has a sufficiently high durability for the collector current operated at the maximum range and a high electric strength between collector and emitter thereof. Furthermore, other circuit components, such as capacitor C9 provided at output rectifying portion 6, should be of a type which can accept a high electric power. Thus, the circuit becomes bulky in size and results in a high manufacturing cost.
Furthermore, according to the prior art power supply circuit, although it may be suited for providing a constant voltage to a load, it is not suitable for providing a constant current to a load, such as required in charging nickel-cadmium battery.